implantable sensor to measure liquor pressure
TRANSCRIPT
The human brain is filled with cerebrospinal
fluid (CSF), which is produced in the ventric-
les. Patients suffering from normal pressure
hydrocephalus (NPH) produce more CSF as
they resorb. This leads to an increase of the
intracranial pressure (ICP) in the skull, what
may cause progressive enlargement of the
head, convulsion, tunnel vision and mental
disability due to shortage of oxygen and nut-
rients. To reduce the intracranial pressure, the
implantation of a drainage system (shunt)
which removes excessive CSF, e.g. into the
abdominal cavity, is necessary.
Development
In corporation with the companies Aesculap
AG and Christoph Miethke GmbH & Co KG
a system was developed for wireless measu-
rements of the cerebrospinal fluid pressure in
the shunt. The system is approved for use as
a long-term implant in humans.
Implantable SenSor to meaSure lIquor preSSure of a VentrIcular DraInage SyStem
Pressure Sensor and Sensor
Readout Electronics
The integrated capacitive pressure sensor of
the transponder ASIC, as shown in Figure 3,
consists of an array of circular pressure sensi-
tive and pressure insensitive elements. The
capacitor of a pressure sensor is formed by a
fixed electrode in the substrate and a second
electrode as a deflectable membrane of poly-
crystalline silicon above. The cavity under the
membrane is obtained by anisotropic etching
and later vacuum sealing. The diameter of
one element is approx. 100µm and the capa-
citive change between minimal and maximal
pressure is approximately 50fF.
A differential C/V converter transforms the
capacitance of the pressure sensor into a
voltage signal. This signal is A/D converted by
a RSD (Redundant Signed Digit) cyclic A/D
converter into a digital 13 × 2 bit data stream.
1 Shunt sensor position in the drainage system.
2 Handheld reader unit measuring.
F R A U N H O F E R I N S T I T U T E F O R M I c R O E l E c T R O N I c c I R c U I T S A N D S y S T E M S I M S
Fraunhofer Institute for
Microelectronic circuits
and Systems IMS
Finkenstr. 61
D - 47057 Duisburg
Phone +49 203 37 83-0
Fax +49 203 37 83-266
www.ims.fraunhofer.de
Contact
Michael Bollerott
Phone +49 203 37 83-227
3 Pressure and temperature transponder ASIC.
4 Typical raw pressure curve of a complete in
metal encapsulated sensor.
The cyclic conversion algorithm is based on
the conventional restoring numerical division
principle. The conversion of the capacitance
is adjustable in offset and gain to compensate
fabrication related tolerances of the pressure
sensor parameters. The C/V converter and
the A/D converter are realized in a switched
capacitor technique. A typical pressure curve
for raw sensor data is shown in Figure 4.
Transponder System
The complete transponder system, as shown
in Figure 2, consists of a passive transponder
equipped with the described pressure sensor
and readout electronics, and a reader device
which powers the transponder by an electro-
magnetic field and receives back information
from the transponder. In this transponder
system, load-modulation is used to transmit
data from the transponder to the reader. The
transmitted data package comprises of the
sensor readout data, calibration data, a
unique identification number and a checks-
um for data validation. The reader unit calcu-
lates the measured pressure and temperature
out of the received data by a fixed formula.
To overcome the damping caused by the
metal encapsulation of the implant the hand-
held reader was designed with a high quality
factor of the transmitter coil, to optimize the
performance regarding reading distance and
battery lifetime.
Note: The illustrations and information pre-
sented herein are not the specifications of
a medical product but examples of a Fraun-
hofer IMS pressure sensor system.
Parameter Value
Sensor transponder ASIC size (l, w, h) 9.4 mm, 2.5 mm, 0.73 mm
Pressure range 733 – 1233 hPa
Pressure sensor accuracy 2.66 hPa
Pressure sensor sample rate 44 Hz
Temperature range 20 – 45 °C
Temperature sensor accuracy 0.5 K
Temperature sensor sample rate 22 Hz
Operation distance 0 – 5 cm
Reader HF-frequency 133 kHz